Comparison of the different proposed sources for the 1755 Lisbon tsunami: modeling in the West Antilles.

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Comparison of the different proposed sources for
the 1755 Lisbon tsunami modeling in the West
Antilles.

• Jean ROGER, M.A. BAPTISTA, H. HEBERT
• ITS 2009, Novosibirsk, Russia

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Introduction

• 1755 reminder concerning earthquake and
  associated tsunami
• ?estimated magnitude 8.5-9.0
• ?lot of casualties (60000 dead
• people) and destructions
• ?waves reaching more than 15 m locally
• (Cadiz Gulf, Morocco)
• ?reports from Morocco to England of tsunami
  arrival
• Reports of waves causing destructions and loss of
  life in the Açores (Andrade, 2006) and in the
  West Indies (Lander et al., 2002)
• ? Source(s) not well established

Gutscher et al., 2002
Source ????
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Contribution of far field effects study
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Historical data in the West Indies

• Several coeval documents from 1755 in about 10
  places in the Caribbean Islands.

• Reports of important inundation in some places /
  with a lot of details

• Be careful concerning the inhabited areas at this
  time compared to todays
• 1745 80000 inhab. Incl. 65000 slaves
• Today around 380000 inhab.

Document-based maregram
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Modelling vs. Historical reports

• Negligible effect on Lesser Antilles
• 10 cm vs. 2-3 m

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Barkan et al. (2008)s source n5
? Shows a wave amplification in the West
Indies Use of a multigrids model ? Increasing
resolution until 40 m
Construction of high resolution grids
? But this source has NO geological evidence
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Zitellini et al., 1999 Baptista et al., 2003
Mw8.5 / geological evidence
2 segments Marques de Pombal Guadalquivir Bank
? Look at the strike of the 2 segments
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Gutscher et al., 2006
Cadiz Wedge / Mw8.3
? Nothing relevant in the West Indies
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The resonance effect
How can we explain such differences between
modelled amplitudes ?

• What ?
• ? phenomenon of trapping and amplification of
  wave energy (Woo et al, 2004)
• Where ?
• ? semi-enclosed water body harbor , lagoon, bay,
  fjord, etc.
• When ?
• ? the period of arriving waves the eigenperiod
  of the water surface of the considered water body
  (Bellotti, 2007)
• Harbor resonance every harbor has a natural
  oscillation mode with eigenperiod depending on
  physical characteristics of the water body (Jansa
  et al., 2007) i.e. its geometry and depth (Monso
  de Prat and Escartin Garcia, 1994)
• Some submarine features can induce resonance
  effects as shelf resonance

Supposed resonance
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Bathymetric grids
High resolution grids (10 m) ? made from
georeferenced and digitized nautical maps
Ste Annes Bay, Guadeloupe
N
Yelles et al., 2009
Roger and Hébert, 2008
N
Palma Harbor, Spain
Jijel Harbor, Algeria
? With respect of harbor structures
? All the grids are adapted to the imbrication
used by our modelling code during calculation
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First step

• ? look at the maximum sea elevation (Hmax) use
  of numerical tide gages

Palma Harbor, May 2003
The most resonant site for tsunami arrivals in
Balearic
If it does not explain the reported
amplifications in specific areas,
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2nd step frequency analysis

• Accurate study of recorded signal ? FFT (Fast
  Fourier Transform)
• ? reveals frequency peaks

Sahal et al., submitted
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• Use of synthetic signals (Ampl. and period known)
• ?Determine harbor eigenperiod
• spectral analysis of tide gage records and
  eigenvalues analysis
• Thus we know what kind of signal is able to make
  the water body to react

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3rd step

• Go back to the geometric parameters of the
  source...
• ? Inverse problem

eigenperiod of harbor (natural and harmonics)
Be careful to other type of resonance
Knowledge of arriving signal
What kind of deformation is able to generate such
signal
Comeback over sources parameters
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Conclusion

• Considering far-field effects could be a good
  contribution to the estimation of the sources
  parameters
• Strike angle not seems to be a major factor to
  wave amplification in the West Indies (? In each
  tested case No main energy paths towards the
  West Indies)
• Underlines the need of high resolution
  bathymetric datasets
• Frequency analysis this method could explain the
  great sea elevation values in Antilles during
  1755 event and allows to select one or several
  possible sources
• We could protect harbors using these eigenvalues
  knowledge (Monso de Prat and Garcia, 1994)
• Today we are not able to reproduce amplification
  exactly

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Perspectives

• Try to find sources parameters able to generate
  far field effect in agreement with historical
  reports and compare them to proposed sources
  parameters, taking into account the eigenperiods
  of studied harbors
• Look at other places concerning by far-field
  effects of this 1755 tsunami and do inverse
  modelling

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Work funded by the French ANR project MAREMOTI
MAREograph and field tsunaMi observations,
mOdeling and vulnerabiliTy studIes for Northeast
Atlantic and western Mediterranean